1. Field of the Invention
This invention pertains to the construction of walls used to contain high energy fuels or the like. More particularly, this invention pertains to such structure which, when subjected to a high temperature environment, vents in a predetermined manner to relieve internal pressure and permit controlled combustion of the high energy fuel contained therein.
2. Description of the Prior Art
Structures which contain high energy fuels, such as rocket motor combustion chamber casings, or warhead casings, will explode when exposed to a fire or other high temperature environment because of uncontrolled ignition of the contained fuel and resultant catastrophic internal pessure buildup. One place where this problem is especially serious is on the flight deck of an aircraft carrier where a large quantity of explosive ordnance may be temporarily stored during flight operations. In such situations it is possible for an aircraft fuel tank to rupture and cause a fuel fire on the flight deck. This fire will engulf ordnance loaded on aircraft or stored nearby, and will eventually cause that ordnance to explode with devastating effect upon surrounding equipment and personnel.
Military test procedures and requirements for the heat resisting capability or ordnance items are described in Mil-Std-1648, a military specification of the United States Government which is unclassified and available to the public.
Past attempts to solve this problem have included placing a thermal barrier on the exterior of the rocket motor casing or warhead in an attempt to thermally insulate the rocket motor propellant or explosive material from the fire. This approach attempts to increase the length of time the ordnance item may be exposed to a fire without exploding by keeping the internal temperature low. If the fire is not extinguished within a short period of time, the internal temperature will increase and the ordnance item will ignite and explode. Such thermal insulation coatings are not efficient because they add nonfunctional weight to the rocket or warhead, and they increase the cost and field handling problems associated with that ordnance item. Also, aerodynamic drag of that ordnance item may be increased.
Pressurization liners have also been used with varying degrees of success. Such liners are designed to degrade into a gas at low temperature to supply controlled internal pressure for causing rupture of preweakened venting structure in the casing wall. The main problem encountered with a pressurization liner is that if the liner degrades and pressurizes too suddenly, the propellant grain may crack and when ignited, the grain will cause an uncontrolled pressure rise which will cause the casing to explode anyway.
Intumescent coatings have also been used with varying degrees of success. The main drawback to using an intumescent coating is that the coating is applied to the exterior of the item, and must be protected from scratches and abrasion. When exposed to a fire, the intumescent coating forms a very light insulating layer which may easily be swept away if, for example, a stream of water is directed upon it.
Some attempts to overcome the explosion hazard when explosive filled containers are subjected to fire or other high temperature environments have involved placing a thermally insulating liner between the inside of the pressurization liner and the high energy fuel. Once again, such an approach may increase the length of time before an explosion occurs, but it will not by itself ultimately prevent explosion if the explosive filled container is not removed from the fire or other high temperature environment. Also, with additional liners, the available volume for high energy fuel is reduced. Finally, existing ordnance may not be retrofitted with extra liners at low cost.